TIANJIN, China (AFP) - Massive explosions at a chemical storage facility in the Chinese port of Tianjin on Aug 12 have killed 112 people and injured more than 700. They also raised fears about toxic chemicals poisoning the air, particularly sodium cyanide.

Here are a series of questions and answers explaining what is known about the blasts, but also the fears over a lack of information about the chemicals that may have been dispersed.

How did the explosions occur?

Firefighters were called to a blaze at a hazardous chemicals storage facility last Wednesday night in an industrial zone of Tianjin, one of China's biggest cities with a population of 15 million people.

Then, two massive explosions took place about 11.30pm local time, sending a giant fireball sweeping across the area.

The authorities have struggled to identify the substances present at the facility and say they do not yet know exactly what triggered the explosions.

But questions have been raised over whether firefighters responding to the initial blaze could have contributed to the detonations by spraying water over calcium carbide, listed as being at the site.

Calcium carbide reacts when wet to produce highly combustible acetylene gas.

How many casualties were there and who were they?

At least 21 of the 112 people confirmed killed were firefighters, many of whom who came to tackle the initial blaze, officials say. Others were migrant workers who lived in dormitories nearby.

Overall, 722 people have been hospitalised, 58 of whom were in critical or serious condition on Saturday evening.

Most of the recovered bodies have yet to be identified and around 85 firefighters remained missing, officials said on Sunday.

Several police officers are also missing, but a police officer quoted in the Chinese media on Saturday said his station's losses had not been included in the toll at that point.

Officials have said 17,000 families were affected by the blast, 1,700 industrial enterprises and 675 businesses.

What was the extent of the damage?

Residents likened the shockwaves to an earthquake, and aerial footage of the blast zone shows scenes of monumental devastation, with buildings burned out, shipping containers crushed and tumbled like piles of children's blocks, and fields of burned-out vehicles.

About 10,000 new imported cars near the blast site were destroyed, according to Chinese media reports, and even buildings 3km away had their windows shattered.

Up to 6,000 people were relocated on Thursday to schools because their homes were damaged by the shockwaves, according to Xinhua.

What chemicals were at the storage facility?

Officials have listed a litany of chemicals that may have been at the hazardous goods storage facility when the explosions happened, but have been unable to say precisely which ones were present.

Potassium nitrate and ammonium nitrate are believed to have been there.

Chinese reports said 700 tonnes of sodium cyanide were at the site, and officials called in experts from producers of the material to help handle it. Hydrogen peroxide, which neutralises it, has been used.

A military team of 217 chemical and nuclear experts was deployed early on, and rescue teams wore protective clothing in the blast area.

What are the concerns now?

That extremely dangerous chemicals could be floating in the air, may still be leaking from the facility or could cause another explosion.

The Chinese authorities have given mixed signals about the threat level.

Officials have insisted that despite the presence of some pollutants at levels above normal standards, the air in Tianjin remains safe to breathe, but some police and other personnel at the scene have been seen wearing gas masks or full protection suits.

On Saturday, a 3km radius from the site of the blasts was evacuated, state-run media reported. Officials said later the reports were inaccurate, but barriers prevented access and people were seen leaving the largely devastated zone.

Xinhua reported late Saturday that cyanide density in waste water had been 10.9 times standards on Thursday.

It had since fallen but was still more than twice the normal limit, it said.

What is sodium cyanide?

Sodium cyanide is a white crystal or powder. It releases hydrogen cyanide gas, which the US Centers for Disease Control describes as "a highly toxic chemical asphyxiant that interferes with the body's ability to use oxygen".

The gas has a distinctive bitter almond odour, it says, but a large proportion of people cannot detect it.

"Exposure to sodium cyanide can be rapidly fatal," it adds.

The chemical has been used in the US to carry out executions in gas chambers.

Sodium cyanide is used in the mining industry to recover gold, among other industrial purposes.

What has been the government's response?

The authorities have struggled to completely extinguish the blaze, which intensified on Saturday, when several fresh blasts were also heard.

They have also moved to limit criticism of handling of the disaster, blocking some critical posts on social media, and suspending or shutting down more than 360 social media accounts.

A total of 50 websites have also been punished for "creating panic by publishing unverified information or letting users spread groundless rumours", according to the Cyberspace Administration of China.

Executives from the company that ran the storage facility, Tianjin Dongjiang Port Rui Hai International Logistics, were quickly detained on Thursday.

Firefighters, military and other rescue workers have continued to search for people still believed to be missing.